The present invention relates to physiological signal processing systems and methods.
Physiological functions typically are monitored by producing signals intended to represent these functions and processing such signals to extract useful data concerning the functions. Non-invasive monitoring techniques for acquiring such signals often provide a reduced risk of infection as compared to techniques which involve acquiring a signal directly from the bodily organ whose function is to be monitored. It is, however, difficult to avoid unintentionally receiving interferring components in such signals due to the functioning of adjacent organs, especially where non-invasive monitoring is undertaken.
For example, the monitoring of respiration by impedance pneumography involves the detection of relatively small changes in transthoracic impedance, which are occasioned both by the expansion and contraction of the lungs and by cardiovascular activity. Typically, a respiration event is detected when the respiration signal crosses a fixed threshold level, which may be adjustable by the user. If the threshold level is set too low, cardiovascular artifact can trigger the detection of a respiration event when in fact no respiration has occurred; apnea, therefore, can go undetected, with potentially severe consequences to the patient. If the threshold level is set too high, normally shallow breathing can go undetected, resulting in a false apnea alarm.
One approach to the solution of these problems assumes that the respiration signal will fall predominately in a range of frequencies below an arbitrarily selected frequency and that the cardiac components will fall predominately above that frequency. Accordingly, the respiration signal is processed by a low pass filter having a fixed cutoff frequency in order to suppress cardiovascular artifact in the respiratory signal. However, both respiration and heart rate can vary widely so that the foregoing assumption can lead to a disproportionately large cardiovascular artifact component in the respiration signal, with the possibility that a true apnea may be missed while the monitor counts artifact as respiration.
Another approach to these problems assumes that cardiovascular artifact will always be less than a certain percentage of the respiration signal. Accordingly, the threshold level is adjusted to a predetermined percentage of the last peak value of the respiration signal. However, large transients which will appear from time to time in the signal will cause an abrupt increase in the threshold level. Typically, the threshold level will be stored in a capacitor which will bleed charge to ground through a resistor to permit the threshold level to adjust downwardly when the peak value of the signal is decreasing. Due to the possibility of large transient pulses, a relatively short time constant must be selected, or false apnea detection may occur. This, however, creates a new problem; namely, the threshold level may fall off so rapidly that cardiovascular artifact may be detected as a respiration event, so that true apnea is missed.
In a modification of the foregoing method, it is assumed that when the detected respiration rate is the same as the heart rate, cardiovascular artifact is being detected by the respiration monitor. Accordingly, the threshold level is abruptly increased by an arbitrary amount when this occurs in an attempt to avoid counting cardiovascular artifact as a respiration event. This likewise requires that the threshold decrease rapidly thereafter so that true respiration can be detected and false apnea avoided. Once again, cardiovascular artifact can be counted as respiration and true apnea can be missed.